Shrinking Brains May Be Cost of Long Life

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Brains that shrink with old age may be the cost of a life span
stretching into the 80s, according to a new study that finds
while human brains get smaller with age, the brains of our
closest primate relatives, chimpanzees, don't lose volume at all.

The findings suggest that the human life span (average length of
life of an organism) isn't just an extended
version of the life span of other mammals, said study
researcher Chet Sherwood, an anthropologist at George Washington
University in Washington, D.C. Instead, humans seem to experience
old age in a unique way.

"The greatest point of deterioration that we found is in that
part of the human life span which is beyond the life span of wild
chimpanzees," Sherwood told LiveScience. "We think that the
effect we see is the result of increased longevity."

The findings, published today (July 25) in the journal
Proceedings of the National Academy of Sciences, could explain
why humans are uniquely vulnerable to Alzheimer's and other
age-related brain disease, Sherwood said. [Read: 7 Ways
the Mind and Body Change with Age ]

Chimps vs. humans

Chimps and humans are separated by between 6 million and 8
million years of evolution, dating back to the last common
ancestor we shared with our ape cousins. The two species have
clearly diverged quite a bit during that time period, with one of
the less-visible differences occurring in the brain. In old age,
the
human brain shrinks in overall size by 10 to 15 percent. In
chimps of equivalently old age, the new study finds, there is no
shrinkage at all.

Sherwood and his colleagues compared magnetic resonance imaging
(MRI) scans of the brains of 87 humans ages 22 to 88 with MRI
scans of 69 chimpanzees from the Yerkes National Primate Research
Center in Atlanta. The chimps ranged from 10 to 45 years old. The
researchers also examined the brains of deceased chimps as old as
51.

In the wild, Sherwood said, chimps are usually dead by 45.
Likewise, studies of foraging communities suggest that the
mid-80s is the typical life span for pre-industrial humans. Thus,
the study captured more than the full range of life spans and
brain aging that you would expect to see in the evolutionary past
of chimps and humans.

Focusing on the frontal lobe and hippocampus, the most
shrink-prone regions of the human brain, the researchers found
the expected loss of brain size with age in the human volunteers.
But the chimps'
brains showed no size change at all.

Long lives, shrinking brains

That's not to say that chimps' brains don't change with age —
other studies show cellular changes and subtle structural changes
in animals as they get older — but the direct comparison makes it
clear that "whatever is going on is certainly more extreme in
humans," Sherwood said.

Sherwood suspects that as natural selection began to favor
the large
human brain, humans had to deal with the associated costs,
including relatively helpless infants and an extended childhood
as that brain develops. In most mammals, females die once they
can't reproduce anymore. But human women can expect to live 40
years past menopause. This
longevity may have evolved so that grandparents could help
their own children with childrearing, thus ensuring that their
offspring's offspring would survive to spread their genes.

"If you're going to take on another 40 years of life span and
these neurons have to function, that ultimately seems like it's
going to be hard to keep up with," Sherwood said.

In fact, humans outlive their reproductive usefulness for so long
that it's amazing their brains don't degrade faster, said Todd
Preuss, a neuroscientist at Yerkes National Primate Research
Center who was not involved in the current study. Preuss and his
colleagues are studying the subtler differences between human and
chimp brains, trying to understand how molecular and cellular
variations separate us from our nearest living relatives.

"There is the opportunity here for much more microscopic and
biochemical work," Preuss told LiveScience. "The particular
vulnerability of the human brain to Alzheimer's disease suggests
that there's something unusual about our brain chemistry."